The dartos fascia is composed of smooth muscle fibers known as the dartos muscle. This fascia is found beneath the skin of the scrotum and outside of the membrane surrounding the testicles. This membrane is known as the external spermatic fascia. This fascia is a continuation of the structure known as Scarpa's fascia, a layer of subcutaneous tissue found in the wall of the abdomen. It is this tissue layer that gives the scrotum its wrinkled look.
The muscle fibers making up the dartos fascia are actually present in both genders, although the role is a bit different in males than it is in females. In males, this fascia is termed tunica dartos and is found under the skin of the scrotum. The dartos muscle begins to lose some of its tone as a man ages. This can sometimes cause the scrotum to hang a bit lower than it once did. The scrotum can also appear a bit smoother due to this loss of muscle tone.
In females, these same muscle fibers are called the dartos muliebris. These fibers are located underneath the labia majora, also referred to as the large lips surrounding the vagina. The muscle fibers are less developed in the female than the corresponding fibers found in the male.
One of the roles of the tunica dartos is to assist in temperature regulation involving the testicles. This promotes a process known as spermatogenesis. In this process, mature sperm cells are developed, allowing for reproduction. This process is made possible when the skin of the scrotum is wrinkled due to the contraction and expansion of the muscle fibers.
When the muscle fibers cause the skin of the scrotum to contract, there is less room available for the loss of heat. This type of muscle contraction reduces the amount of heat that is lost and works to keep the testicles warm. When the muscle fibers expand, the surface area of the scrotum is increased. This action increases heat loss and works to cool the testicles.
The cremaster muscle covers the testicles. This muscle works along with the dartos muscle to elevate the testicles. This action is different from what is known as the cremasteric reflex. With this reflex, the scrotum is elevated when a particular part of the inner thigh is touched. Testing this reflex can help medical professionals detect an emergency situation, particularly when there has been pain associated with the testicles or a traumatic injury involving the groin area.